Since electromechanical switches such as MEMS switches are expected to have superior properties as compared with GaAs FET switches or PIN type diode switches, broad researches are being done to apply the MEMS switches to radio communication systems. The MEMS have heretofore come to the fore due to their low loss, good isolation, low power consumption, good linearity, miniaturization, and capability of high integration. However, there has been a problem that the MEMS switches are prevented from being put into practical use, due to their high driving voltage, low operating speed, insufficient reliability, etc.
Generally, a capacitive coupling type MEMS switch is constituted by a fixed electrode, a movable electrode disposed opposite to the fixed electrode, and a dielectric deposited on the movable electrode and/or the fixed electrode. Due to a voltage applied between the movable electrode and the fixed electrode, an electrostatic force is generated to attract the movable electrode to the fixed electrode. Thus, the distance between the electrodes is changed. When the distance between the electrodes is changed, the capacitance, that is, the impedance is changed so that a signal can be turned ON/OFF. Due to the dielectric formed between the movable electrode and the fixed electrode, the coupling is not resistive but capacitive.
In order to obtain a low-loss MEMS switch, it is necessary to reduce the impedance when the MEMS switch is ON. In order to obtain sufficient isolation, it is necessary to increase the capacitance change ratio. This capacitance change ratio can be approximated by the following expression:CON/COFF=(e0*e*Aoverlap/ddiel)/(e0*er*Aoverlap/dair)=dair /ddiel′where dair and ddiel designate the thicknesses of the air gap and the dielectric, er designates the dielectric constant of the dielectric, and Aoverlap designates the area of a coupling region of the movable electrode.
One of problems of a capacitive switch is reduction in capacitance change ratio caused by the surface roughness of electrodes. When the surfaces of the electrodes to abut against each other have undulate shapes, a protrusion portion abuts against a protrusion portion so that the distance between the electrodes cannot be reduced sufficiently with respect to the surfaces as a whole. Thus, there has been a problem that the capacitance change ratio is reduced.
Therefore, J. Park et al. has proposed not a structure in which an electrode formed out of (metal-dielectric) is brought into contact with an electrode formed out of metal, but a structure in which an electrode formed out of (metal-dielectric-metal) is resistively coupled with an electrode formed out of metal. According to this structure, even if the surface accuracy in a metal layer is not sufficient, an insulating layer will be formed along the surface of an electrode when the electrode is formed. Further, a metal layer will be formed along the insulating layer. Thus, the substantial distance between the electrodes can be reduced without being affected by the surface accuracy.
There has been proposed another MEMS switch using a single metal layer and assembled to be displaced in a plane parallel to a substrate surface (Patent Document 1). This MEMS switch is constituted by at least one air bridge including a movable electrode disposed adjacently to a fixed electrode. A movable electrode having a three-layer structure made of metal layers with a dielectric layer formed in the coupling surface. The dielectric layer is, for example, a silicon oxide film, a silicon nitride film, or the like. This movable electrode is driven by an electro static force so as to be displaced in a plane parallel to the substrate surface. In this structure, the electrodes can be formed out of a single metal layer because the movable electrode is driven in a plane parallel to the substrate surface. However, the contact is based on metal-to-dielectric coupling.
Further, there has been proposed not an MEMS switch in which a movable contact itself is driven but an MEMS switch in which a beam connected to the movable contact is driven by a driving electrode provided on the substrate surface (Patent Document 2).    Non-Patent Document 1: J. Park et al., “Electroplated RF MEMS Capacitive Switches” IEEE MEMS 2000    Patent Document 1: U.S. Pat. No. 6,218,911B1    Patent Document 2: JP-A-2003-71798